Follow-up Testing of Blood Donors Found to Be Enzyme Immunoassay Positive/Western Blot Negative for HTLV-III Antibody

To the Editor.—  The human T-cell lymphotropic virus type III antibody enzyme immunoassay (EIA) has now been used for approximately eight months in blood centers. Early experience with the test nationwide shows 0.25% of the normal blood donor population to be positive.¹ Of EIA positives, 23% are subsequently found to be Western blot (WB) positive.²The EIA is a spectrophotometric assay that is interpreted based on optical density values compared with a positive cutoff. Studies have correlated optical density values of EIA-positive blood samples with WB positivity and with positive viral cultures; most lowoptical density EIA-positive blood samples (close to the cutoff) are false-positives, ie, WB negative and viral culture negative.³ Enzyme immunoassay-positive/WB-negative donors are equally likely to be female as male, whereas most EIA-positive/WB-positive donors are males and many are members of high-risk groups for acquired immunodeficiency syndrome (AIDS).²Blood centers are currently discarding EIA-positive/WB-negative